Plasma reduction processing of materials

ABSTRACT

In a process for the reduction of a metalliferous ore or concentrate the ore or concentrate is first prepared into a particulate form. A reaction chamber ( 3, 103, 203, 301, 401, 503, 603, 702 ) is then charged with ore or concentrate, a reductant and an input gas. The reaction chamber ( 3, 103, 203, 301, 401, 503, 603, 702 ) is irradiated with electromagnetic radiation within a frequency range of  30  MHz to  300  GHz until a non-equilibrium plasma is initiated. The plasma is sustained and controlled with the radiation until the ore or concentrate is reduced to form reduction product.

FIELD OF THE INVENTION

The present invention relates to the chemical processing of materials ina plasma environment, and in particular relates to pyrometallurgicalreduction processes in a plasma environment.

BACKGROUND OF THE INVENTION

The pyrometallurgical reduction of metalliferous ores and concentratestypically involves the heating of the ore or concentrate in a smeltingfurnace with a reductant to a temperature which generally melts the oreand at which chemical reaction of the ore/concentrate with the reductantreduces the ore/concentrate into metallic product or higher end-valueproduct with a lower oxidation state. Large amounts of energy arerequired to initiate and sustain reduction processes in such smeltingfinances, and the recovery rate of metallic product often renders suchoperations commercially unviable. The non-reduced components of theore/concentrate form a slag, which often contains valuable metalliccontent. Recovery of the metallic content from such slags is, however,again often commercially unfeasible by conventional methods.

Microwave radiation has been utilised in various industrial applicationsfor the application of energy to heat materials, including the microwaveheating of chemical reactants to kinetically and thermodynamicallystimulate the same for the initiation of chemical reactions. Microwavetreatment of metalliferous ores and other comparable materials has beenutilised as an augmentative precursor treatment, applying energy to theore to thermodynamically stimulate the same and prepare it forconventional recovery techniques such as conventional pyrometallurgicalreduction, leaching or hydrometallurgical recovery processes.

OBJECT OF THE INVENTION

It is the object of the present invention to provide an improvedpyrometallurgical reduction process.

SUMMARY OF THE INVENTION

In a broad form the present invention provides a process for thereduction of a metalliferous ore or concentrate comprising the steps of:

-   -   preparing said ore or concentrate into a particulate form;    -   charging a reaction chamber with said ore or concentrate, a        reductant and an input gas;    -   irradiating said reaction chamber with electromagnetic radiation        within a frequency range of 30 MHz to 300 GHz until a        non-equilibrium plasma is initiated, and    -   sustaining and controlling said non-equilibrium plasma with said        radiation until said ore or concentrate is reduced to form        reduction product.

Typically, pressure within said reaction chamber is maintained below 300kPa during irradiation thereof.

Typically, said pressure is also maintained above 40 kPa.

In several embodiments, said pressure is maintained at about atmosphericpressure.

Said plasma may be initiated in said input gas.

Alternatively or additionally, at least part of said input gas may bedecomposed during said irradiation, said plasma being initiated at leastin part in the decomposed product of said input gas.

The reductant will typically comprise a carbonaceous material.

The reductant may include a particulate carbonaceous material blendedwith said ore or concentrate.

The reductant may include carbon monoxide gas, said input gas includingsaid carbon monoxide gas, said plasma being initiated at least in partin said carbon monoxide gas.

The reductant may comprise carbon monoxide gas and a particulatecarbonaceous material.

Alternatively, the reductant may comprise a reactive metal.

The input gas may include an inert gas.

The inert gas may comprise argon or nitrogen.

The input gas may comprise air.

The reductant may include methane.

Preferably, said radiation is microwave radiation.

The ore or concentrate may be a concentrate derived directly from minedore.

Alternatively the ore or concentrate may be a non-ore derivedconcentrate. Said non-ore concentrate may be a residue derived, wastederived or mining derived concentrate, such as from mine tailings orconcentrator residue.

The ore or concentrate may be a concentrate in the form of a residue,such as a slag, slurry or slime, derived from metallurgical processingoperations. Such residue may be derived from pyrometallurgical,hydrometallurgical, chemometallurgocal or electrometallurigealprocessing stages during primary, secondary and/or tertiary stages ofmetallurgical processing operations.

The reaction chamber may be in the form of a fluidised bed reactor.

The reaction chamber may alternatively be in the form of an oven, saidore or concentrate being charged into a crucible placed within saidoven.

The reaction chamber may be in the form of a rotary kiln reactor.

The reaction chamber may be in the form of a cyclone reactor.

The reaction chamber may be in the form of a conveyor fed reactor.

In such a conveyor fed reactor, said ore or concentrate is preferablyprepared into a pelletised particulate form.

Preferably, said reduction product is of metallic form.

Said metallic reaction product may be in the form of a fume, said fumebeing extracted from said reaction chamber and separated from gasesproduced during said reduction.

Alternatively, said reduction product is a compound of reduced oxidationstate.

The reduction product may be formed by reduction of said ore orconcentrate through a series of subsequent reduction reactions.

The process may include the step of generating carbon monoxide, saidplasma being initiated and sustained at least in part in said carbonmonoxide.

When said input gas includes air and said reductant includes particulatecarbonaceous material, said carbon monoxide may be generated fromreaction of oxygen within said air with said particulate carbonaceousmaterial.

Alternatively or additionally, when said reductant includes particulatecarbonaceous material, said carbon monoxide may be generated fromreaction of carbon dioxide produced during said reduction with saidparticulate carbonaceous material.

Alternatively or additionally, particulate carbonaceous material may beintroduced into said reaction chamber after initiation of said plasma,said carbon monoxide being generated from reaction of carbon dioxideproduced during said reduction, and/or oxygen within said air when saidinput gas includes air, with said introduced particulate carbonaceousmaterial.

Preferably, said ore or concentrate is enveloped in a non-oxidising orinert gas environment during said reduction and during cooling of saidreduction product following irradiation of said reaction chamber.

Preferably, said non-oxidising or inert gas is introduced to saidreaction chamber during said cooling.

In one embodiment, said input gas is passed through said ore orconcentrate during said irradiating step.

Preferably, said input gas is blasted upwardly through said ore orconcentrate.

Preferably, said input gas is preheated prior to charging into saidreaction chamber.

It has been a commonly held view that the generation of plasmas duringthe microwave chemical processing of materials, and in particular duringthe pyrometallurgical reduction of metalliferous ores and concentrates,is detrimental to the process system hardware and monitoring and controldiagnostics equipment, and accordingly it is typical for such processesto be controlled in a manner to explicitly avoid the generation of aplasma.

Reaction rates, however, can increase by one or more orders of magnitudeunder plasma processing. A plasma is a mixture of excited molecules,atoms, ions, electrons and recombined particles in a ground state hostgas. With the high particle energies which are characteristic of suchplasma components, the physical and chemical behaviour of thesecomponent particles differs markedly from equivalent particles in the“ground state”.

In pyrometallurgical processes conducted in a plasma environment, thereis a predominance of reaction chemistry occurring at the plasma-solid orplasma-liquid interface. Whilst this feature is characteristic ofpyrometallurgical processes in general, reaction rates across theseinterfaces are greatly enhanced by plasma chemistry, with an abundanceof highly energised reactive species.

Plasmas initiated and sustained at “high” pressures exhibit anapproximate equivalence of temperature between electrons and heavyparticles (ions, atoms, excited molecules). Accordingly these plasmasare termed equilibrium plasmas, as there is (approximate) thermalequilibrium between particles. This is exhibited particularly at higherpressures as the high density of particles provides an increasedfrequency of collision between particles distributing energy relativelyevenly between particles, providing a consistent bulk temperaturethroughout the particle species of the plasma. Because of thehigh-energy densities (thermal mass), equilibrium plasmas have commonlybeen utilised as precursor methods in material processing for theircapability to heat, sinter, melt or vaporise solid materials. These areall essentially physical processes merely taking advantage of thephysical thermal kinetics (properties) of the equilibrium plasma.

Non-equilibrium plasmas, which are more characteristic of low pressureenvironments, are characterised by particle temperature non-equivalence,with the “temperature” of electrons far exceeding that of thetemperatures of the heavier particles. FIG. 1 depicts the separation ofelectron and heavy particle temperatures at low pressures, both withconventional plasmas and plasmas stimulated by microwave (or RF)radiation. It can be seen that at higher pressures, the electron andheavy particle temperatures merge. In a non-equilibrium plasma, thephysical and chemical behaviour of the component particles may beprofoundly different from that in the equivalent ground stateenvironment. In a non-equilibrium plasma, with the various particlespecies moving with different energies, the measure of such energy,typically in the form of a “temperature” will vary greatly betweenspecies and between particles in each species population. This isevident when “temperature”, a measure of thermal energy, is obtained bya mean reading by averaging-out the electron voltages (temperatureequivalents) of particles having no adjustment for “thermal mass”.Accordingly, the temperature of the plasma itself becomes meaningless asparticle “temperatures” vary by perhaps four orders of magnitude, and“bulls” temperature measurements of plasma by different methods candisagree by an order of magnitude.

The processing effectiveness of low pressure, non-equilibrium plasmas isimbued by the reactivity of the chemically active species present ratherthan by the total energy available in the plasma This reactivity makesnon-equilibrium plasmas more suited to systems reliant upon the chemicalkinetics of the chemical reactions, as per that of the presentapplication, than the equilibrium plasmas which have been used primarilyin physical processes as discussed above.

The form of the diagram of FIG. 1 will be dependent upon variousparameters, including the gas composition, ionising characteristics ofthe species present, and the form of energy applied to the system togenerate the plasma. The pressure up to which a plasma will be of thenon-equilibrium form will thus vary depending on these and otherparameters.

Typical methods of producing plasmas are through ionisation by heating(thermal stimulation), ionisation by irradiation, and ionising byelectrical discharge. Whilst most plasma production methods will resultin an equilibrium plasma at pressures up to around atmospheric pressure,it is believed that the generation of a plasma by irradiation,particularly in the RF and microwave frequency ranges between 30 MHz and300 GHz, pushes the graph of FIG. 1 to the right as depicted, such thatnon-equilibrium plasmas can be generated and sustained at operationallyimportant pressures around atmospheric (101.4 kPa) and up to about threeatmospheres (about 300 kPa) under sufficient applied energy, appropriateavailable species (chemistry) and at responsive radiation frequencies.

This is believed to be as a result of the microwave radiation applyingenergy to the dieletrically disparate particles of the plasma, inparticular to the electrons. At frequencies within the RF and microwavefrequencies, only the electrons in the ionised field can follow theoscillations of the electric field applied. As a result the electronsbecome more highly energised that the heavier particles of the plasma,such that the RF/microwave plasmas can generally be defined asnon-equilibrium plasmas. Such RF/microwave plasmas can be induced andoperated over a large pressure range, from below 0.1 kPa (for operationsoutside the main interest of the present invention), to pressures inexcess of 300 kPa.

When a microwave field is applied across a gas, charged particles inthat gas are accelerated. Because the mass of electrons is much muchless than that of the heavier ion, atom and molecule particles, theaction of the field is primarily to give energy to the electrons.Accordingly, electron temperatures can be in the extremely high range oftens of thousands of kelvin whereas the apparent bulk temperature of theplasma (primarily determined by the heavier particles) is orders ofmagnitude lower.

Reaction rates are generally governed by the mass transport diffusion ofreactants, which is greatly enhanced by dielectric heating mechanismsduring RF/microwave processing, typically in the presence of anRF/microwave stimulated plasma which, by definition, will have a highpopulation of reactive species.

Plasma processing utilising RF/microwave stimulation also enables agreat degree of control over the process, with the microwave radiationable to be directed to the reactant charge, in such a way as to envelopethe entire reactant charge within the reaction chamber or to occupy azone discretely within the charge. In continuous processing systems,residence time and thermochemical parameters can effectively becontrolled through control of the applied radiation, providing superiorprocessing or reduction results.

Whilst lower pressures well below atmospheric pressures ensuregeneration of an unambiguously non-equilibrium plasma with a largedisparity between the temperatures of the electron and heavierparticles, if the pressure in the reaction chamber is too low, then thedensity of reactive species to carry out the chemical processing will betoo low for economically viable processing. Accordingly, it is preferredthat the pressure of the reaction chamber in which the plasma isinitiated and sustained is greater than 40 kPa.

The inherent advantage of the non-equilibrium plasma chemistry(ionisation chemistry) of non-equilibrium plasmas when utilised inchemical and metallurgical applications is that these plasmas canprovide particles with the high energy required to stimulate andcomplete chemical reactions at high kinetic rates. For the range ofapplications relevant to the present application, high rates of masstransfer are desired with the high kinetic rate. Therefore, commerciallyviable productivity levels are often not achievable at extremely lowpressures which provide extremely low rates of mass transfer.

Conversely, the advantage of processing certain reactions undernon-equilibrium plasma conditions, despite low mass transfer rates, isthat in the low density plasma environment, the high energy freeelectrons and ionised particles experience a greatly increased mean freepath before collision and re-combination, imparting greatly increasedenergy to re-combination chemistry. This increased energy at possiblereaction sites enables the activation energy requirement to be met forreactions which require extremely high energy input to proceed.Consequently, certain thermodynamically demanding metallurgical andchemical reactions can be carried out efficiently, if slowly, or if atall, by utilising the extremely high energy particles at low pressures.

Processes which require protection from re-oxidation reactions benefitfrom the protection implied by removal of potential oxidation sources byinitial and continuing evacuation of oxidising agents, such as thecommon reduction reaction product carbon dioxide, from the reactionenvironment. This can be achieved by maintaining the process at lowpressures, continually evacuating the reaction chamber. Alternatively,or additionally, such carbon dioxide can be converted to the reductantcarbon monoxide with fine carbon in the reaction chamber at elevatedtemperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred forms of the present invention will now be described by way ofexample with reference to the accompanying drawings wherein:

FIG. 1 is a diagram showing the separation of electron and heavyparticle temperatures in a plasma at varying pressures.

FIG. 2 is a partially cross sectioned view of a reaction chamber used inthe process of Example 1.

FIG. 3 is a partially cross sectioned view of a reaction chamber used inthe process of Example 2.

FIG. 4 is a partially cross sectioned view of a reaction chamber used inthe process of Example 3.

FIG. 5( a) is a partially cross sectioned view of a reaction chamberused in the process of Example 4.

FIG. 5( b) is an enlarged fragmentary view of the top portion of thereaction chamber of FIG. 5( a).

FIG. 5( c) is a fragmentary cross sectional view of the gas generationsystem associated with the reaction chamber of FIG. 5( a).

FIG. 5( d) is a cross sectional view of the reaction chamber of FIG. 5(a) taken through section 5—5.

FIG. 6 is a partially cross sectioned view of a reaction chamber used inthe process of Example 5.

FIG. 7 is a partially cross sectioned view of a reaction chamber used inthe process of Example 6.

FIG. 8 is a partially cross sectioned view of a reaction chamber used inthe process of Example 7.

FIG. 9( a) is a cross sectioned view of a reaction chamber used in theprocess of Example 8.

FIG. 9( b) is a cross sectional view of the reaction chamber of FIG. 9(a) taken through section 9B—9B.

FIG. 9( c) is a cross sectional view of the reaction chamber of FIG. 9(a) taken through section 9C—9C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

This example details a process to reduce monazite [(Ce,La,Th) PO₄],using an incrucible batch reduction process, to eradicate the phosphorus(of the phosphate) and concentrate the reduced heavy metals into onemetallic or carbide product. With the phosphorus removed, the reactionproduct heavy metal (carbide) concentrate is suitable for furtherextractive processing in a halogenation, fractional distillation thendissociation route. The monazite used in this example containedLa;Ce;Th; in approximate atomic ratio of 3:1:1. Other phosphate mineralshave also been processed in a similar manner with comparable outcomes.The apparatus utilised to carry out the process of this example isdepicted in FIG. 2.

Firstly, the monazite concentrate, which had been derived form mineralsands, was prepared in a particulate form by milling to a grain size ofless than 10 micrometers, and intimately blended with a 10 percentstoichiometric excess of a reductant in the form of fine pure carbon.The fine blend increases the available reaction interface area.

100 grams of the blended monazite and carbon was charged into a lowdensity alumina crucible 1 (see FIG. 2). The crucible 1, being formed oflow density alumina is microwave transparent. The monazite/carbon blendwas charged loosely into the crucible 1 without packing to maximise itspermeability.

A microwave transparent aluminium silicate based fibre-mat insulationwrap 2 covered the exterior of the crucible, insulating the same so asto maintain heat within the crucible 1. A partially open insulation lid2 a was placed over the opening of the crucible to insulate the samewhilst allowing for the escape of gases and observation of the cruciblecontents. The insulation wrapped crucible 1 was then placed in areaction chamber 3, in the form of a purpose-built evacuable reactionchamber capable of operation from an effectively “full” vacuum of lessthan 0.1 kPa to 1000 kPa (approximately ten atmospheres). The crucible 1was placed on a microwave transparent refractory brick spacer 4 so as toposition the monazite/carbon blend load toward the centre of thereaction chamber 3 so as to optimise its location within the appliedmicrowave field and thereby optimise its load potential.

The reaction chamber 3 was then sealed with a lid 5. An o-ring 6 withthe addition of vacuum grease was used to seal the joint between thereaction chamber upper flange 7 and the lid 5. The flange 7 and lid 5were then externally clamped utilising a suitable clamp 8. A viewingport 20 was provided in the lid to enable visual monitoring of theprocess.

The sealed reaction chamber 3 was then evacuated via reaction chamberoutlet 9 utilising a suitable vacuum pump. The reaction chamber 3 wasevacuated to the system dependant pump limit of less than 1 kPa, asmonitored on a vacuum gauge 10. The reaction chamber 3 was then chargedfor two minutes with high purity argon gas via gas inlet 11. Theevacuation/charge cycle was then repeated three times to ensuresubstantially all air within the reaction chamber had been replaced withthe argon gas. Removing the air ensured all oxygen had been removed fromthe reaction chamber, leaving an inert atmosphere protecting reductionproduct from re-oxidation.

The argon supply was then turned off, and the reaction chamber 3evacuated to a minimum pressure of 40 kPa for the reduction operation,again with the system pressure being monitored via the vacuum gauge 10for stability over a 5 minute period.

The reaction chamber was then irradiated with microwave radiation, witha power of 1 kW and a frequency of 2450 MHz, via a top-mounted waveguide 12. The wave guide 12 was arranged with a microwave transparentceramic window 13, formed of alumina, at the interface with the reactionchamber 3 to seal the same and to insulate against radiant heat.

The remnant argon gas (at 40 kPa) in the largely evacuated reactionchamber 3 was the ideal environment for the stimulation of anon-equilibrium plasma capable of initiating the initial solid statereduction of the monazite utilising the carbon as reductant, producingcarbon monoxide (CO) as a by-product of this initial solid statereduction. This initial solid state reduction can be represented byEquation 1(a) below:(Ce,La,Th)PO₄+4C

(Ce,La,Th)OC+P+3CO

  1(a)

The CO produced in the above solid state reduction itself becomes aneffective reductant, transferring carbon in a gaseous form to thereaction interface with greater efficiency. Significantly also for thekinetics of the reaction, the gaseous CO ionises in the microwavestimulated environment to augment the plasma and provide highly activepositive ion species (principally CO⁺) which are not present in theoriginal argon plasma. Whilst the argon plasma exhibits highly energeticnegative (including electrons), positive, re-combined and excitedspecies, it provides no reactive radical species. The ionisation of theCO and the gaseous phase reduction of the monazite can be represented byequations 1(b) and 1(c) below:CO

CO⁺+e  1(b)(Ce,La,Th)PO₄+3CO

(Ce,La,Th)OC+P+3CO₂

  1(c)

The CO in Equation 1(c) may be in the ionised form CO⁺.

A further final reduction step from the oxycarbide to a (complex) metalcarbide was exhibited, again using the CO (at least partly ionised) asreductant. The following Equation 1(d) can be used to reasonably explainthis final step of the reduction process:x(Ce,La,Th)OC+yCO

(Ce,La,Th)_(x)C_(Y)+nCO₂

+mCO

  1(d)

Again the CO in Equation 1 (d) may be in the ionised form CO⁺.

Whilst this reduction to the carbide was confirmed through analysis, thevery hot carbide showed a propensity to strip oxygen from the otherwisevery stable oxide crucible (and other refractory material in contact)and return much of the carbide product material to the more stableoxycarbide phase(s).

During the above reduction reactions, gases produced were drawn away andpumped from the chamber via the outlet 9 during ongoing evacuation ofthe reaction chamber 3 such that the reaction chamber pressure alwaysremained below 50 kPa (absolute). With this low pressure having beenmaintained throughout the process, the plasma sustained can be wellassumed to have remained well within non-equilibrium conditions.

The microwave radiation sustaining the plasma and the reductionreactions was continued until the distinctive CO plasma colour could nolonger be visually detected at the same intensity. This change in plasmacolour and intensity suggests that CO was no longer being produced,indicating that reduction had finished, along with an associatedreduction in pressure back down to the pump limit. At this stage theplasma is expected to have been a principally CO plasma, with the argonhaving largely been flushed through the system during the constantevacuation through the reaction chamber outlet 9.

One minute beyond this visually assessed point of reduced CO plasmacolour and intensity discussed above, and about 10 minutes after plasmainception, the microwave power was shut off. Argon was bled into thestill evacuating chamber via the gas inlet 11, and the pressurestabilised at 20 kPa (absolute) for one hour (to include principalcooling through solidification). The vacuum pump was then disengaged andthe reaction chamber 3 sealed from the pump and backfled with highpurity argon to a modest positive pressure and kept thus until fullycooled before opening on the following day.

The reactor chamber 3 was slowly brought to atmospheric pressure,carefully opened so that no reaction product was disturbed, dislodgednor contaminated, and the crucible 1 removed from the chamber 3. Thecrucible 1 contained the reduced heavy metals carbide (oxycarbide)product, with ash and gangue slag atop. The heavy metals content of thecrucible 1 was scraped from the crucible wall and kept for analysis andfurther processing as desired.

The metal reaction chamber wall 1 and metallic solidification baffles14, which protect the reaction chamber outlet 9 (forming the vacuum pumpinlet) by collecting condensate of the hot vapour phases before theyescape through the outlet 9, were copiously coated in “fluffy” labilephosphorous. The recovered phosphorus was analysed as pure, elementalphosphorous as anticipated by Equations 1(a) to 1(d). The reactorcomponents were then cleaned of reaction products in preparation forfurther batch processing.

EXAMPLE 2

This example details a process to economically recover metals of valuefrom metallurgical wastes and slags using an in-crucible batch reductionprocess. Zinc was recovered from a zinc-bearing slag by reducing themetal in situ in the slag and recovering the metal as metallic fume fromthe hot reacting bed. The zinc fume may, at this point, be re-oxidisedto a refined grade of zinc oxide powder, or reacted with a halogen toyield a zinc halide. In the context of this specification, a fume is tobe understood as including a metallic vapour or a metallic oxide,metallic halide or other similar vapour derived from the metallicvapour.

The process was performed successfully at atmospheric pressure, in a gasmix of nitrogen and carbon monoxide, as opposed to the reduced pressureof Example 1. As metals such as zinc are less of an “oxygen getter” thanthe “reactive metals” (such as the La, Ce and Th of the reduced solidproduct phase of Example 1), the reduced zinc product of the presentExample had a lesser tendency to re-oxidise, and hence required lessprotection against re-oxidation. Consequently, the process could becarried out successfully at atmospheric pressure as the propensity ofthe reduced product to re-oxidise was overwhelmed by the reducingconditions in the reaction environment of the reaction chamber. Thedesired fume product (metal, oxide or halide) dictated the compositionof, and the related chemical possibilities for, the reaction chamberenvironment in which the zinc was reduced and fumed. In the presentexample, process efficiency and product quality were able to bemaintained by generating and sustaining a non-equilibrium plasma atatmospheric pressure, and hence the difficulty and expense of obtainingand controlling a reduced pressure reaction chamber environment wereavoided.

The apparatus utilised to carry out the process of this example isdepicted in FIG. 3.

Zinc-bearing metallurgical slag material having a mineralogical contentof zinc oxide (ZnO), or a more complex mineralogy with ZnO-equivalent,was ground into particulate form to a grain size of less than 500micrometers and blended, to twice the stoichiometric requirement (withrespect to the ZnO), with a reductant in the form of fine reactivecharcoal.

100 grams of the blend was charged into an alumina crucible 101. Thecharge was loosely packed to maximise the permeability thereof. The baseof the crucible 101 was configured with fine passages 101 a passingtherethrough rendering the base porous to allow an updraught of gasesthrough the loosely packed charge of slag and charcoal. The crucible wasmounted on a rigid ceramic box 114 having an open top communicating withthe passages 101 a of the crucible base. The crucible 101 and box 114were insulated with a microwave transparent aluminosilicate basedfibre-mat wrap 102 to insulate the crucible 101 from heat loss. Theinsulated crucible 101 and box 104 were then placed into a reactionchamber 103, in the form of a purpose modified commercially available1300W microwave oven. The crucible/box arrangement was placed on asuitable refractory brick spacer 104 to locate the charge toward thecentre of the reaction chamber 103, and hence favourably placed withinthe applied microwave field. The opening of the crucible 101 was partlycovered by a loose, microwave transparent insulation lid 102 a to allowthe escape of fume reaction product whilst maintaining much of the heatwithin the crucible 101. A viewing port 120 in the roof of the reactionchamber 103 allowed for visual monitoring of the reaction process.

The reaction chamber 103 was closed and then simultaneously evacuatedvia an outlet 109 by a roughing pump whilst nitrogen gas was bled intothe chamber 103 via a primary gas inlet 111. After five minutes, theroughing pump was closed-off and the nitrogen supply was increased to apositive pressure to flush-out and fill the chamber 103.

The chamber 103 was not inherently airtight, and hence the pressurewithin the reaction chamber 103 remained at close to atmosphericpressure. After five minutes of flushing, the primary gas inlet 111 wasclosed. A reductant gas mixture of 20% CO in nitrogen was supplied at alow flow rate to the box 114 via a secondary gas inlet 115 communicatingtherewith through the bottom of the reaction chamber 103. The supply ofreductant gas to the box 114, at a positive pressure, resulted in thereductant gas passing through the passages 101 a in the base of thecrucible 101 and permeating through the slag/charcoal charge.

The reaction chamber 103 was then irradiated with microwave radiation offrequency 2450 MHz and applied power of 1300 W, via two counterposedwaveguides 112 sealed from the reaction chamber 103 by microwavetransparent ceramic windows 113. After several minutes of irradiation,heating the slag/charcoal charge and gases within the reaction chamber,random thermal runaway in disparate, dielectrically disposed particlesinitiated the generation of a CO/N₂ plasma in and above the crucible 101within the reaction chamber 103. This plasma could be observed throughthe shielded viewing port 120.

By operator interpretation of plasma extent and radiant heat intensity,microwave inadiation of the reaction chamber 103 was continued with theapplied power being manually adjusted to provide apparent thermalconstancy and to avoid overheating and failure of the crucible 101 bymelting. From prior experience and the examination of, and knowledge ofthe melting points of phases present and from reaction thermochemistrydata, it was estimated that the process was operated at “temperatures”equivalent to the range 900° C. to 950° C. As previously discussed, theconcept of temperature in a dynamic thermal system such as anon-equilibrium plasma is relatively meaningless, and accordingly“temperature” measurement by thermocouple or direct line-of-sightpyrometry methods is impracticable and would provide almost meaninglessinformation.

Approximately one to two minutes after plasma initiation, a metallicfume was readily detected rising from the plasma, indicating reductionof the zinc oxide content of is the slag. The solid and gaseous statereduction of the zinc oxide, utilising the charcoal and CO plasma asreductants respectively, can be represented by Equations 2(a) and 2(b)below:ZnO+C

Zn+CO

  2(a)ZnO+CO

Zn+Co₂

  2(b)

The CO in Equation 2(b) may be in the ionised form CO⁺.

The metallic fume is particularly easy to visually detect if it has beenallowed to reoxidise as it leaves the reducing atmosphere of thecrucible 101, after having been separated from the slag by the reductionprocesses of Equations 2(a) and 2(b).

To produce a finely divided zinc oxide (ZnO) powder oxide, an oxygen(O₂) stream was introduced such that the reduced zinc metal vapourpassed through the O₂ stream, rapidly converting it to a solid zincoxide phase fume which could be easily collected. Whilst simply passingthe zinc metal vapour through CO₂ already within the reaction chamberenvironment as a reduction by-product of Equation 3(b) also had theeffect of oxidising the zinc vapour, this reaction is less spontaneousand resulted in some of the zinc fume remaining unconverted as solidzinc fume. To produce zinc chloride (ZnCl₂), Cl₂ gas can be introducedacross the hot zinc vapour. The zinc chloride produced had to be cooledsignificantly before a solid fume product could be collected byprecipitation onto a cool surface. The re-oxidation processes can berepresented by Equations 2(c) to 2(e) below:2Zn+O₂

2ZnO

  2(c)Zn+CO₂

ZnO+Co₂

  2(d)Zn+Cl₂

ZnCl₂

  2(e)

The fume product, in the form of metallic zinc, zinc oxide or a zinchalide dependant on system atmosphere, was extracted away through amicrowave transparent borosilicate fume hood 116 placed over thecrucible 101 by an extractor fan to a precipitation and bagging system,via a vacuum seal tap, where the fume was collected as solid fines.

Once the fuming had died away to a visually imperceptible quantity, theprocess was deemed to have finished and irradiation ceased. Completionof the process was later confirmed by analysis of the slag materialremaining in the crucible.

Immediately after the irradiation had ceased, the contents of thecrucible remain reactive and at a high temperature for a prolongedperiod, bleeding of the reducing CO/N₂ gas mixture through the cruciblewas continued until the charge cooled to about 200° C. Continuedbleeding with nitrogen was then used to cool the system to ambienttemperature.

EXAMPLE 3

This example details a process to reduce chromite (FeO.Cr₂O₃) oreconcentrate using an in-crucible batch reduction process resulting in achromium iron alloy. The apparatus utilised to carry out the process ofthis example is depicted in FIG. 4.

The process was carried out at atmospheric pressure, which provedadequate for this example. Further, rather than charging the reactionchamber with a gas mixture of nitrogen and carbon monoxide as perExample 2, air (composed principally of N₂, O₂) was utilised as theinitial gas in the reaction chamber. Combustion of char through heatingand micro-arcing of the char in the oxygen rich environment to produceCO was sufficient to protect against re-oxidation of reaction product.Further, molten slag covers the reduced metallic product phases toconfer further protection in this example, enabling the simpler and moreeconomical processing option of an atmospheric pressure air environment.

High grade chromite ore concentrate was ring-milled with a reductant inthe form of brown coal char in stoichiometric quantity to a grain sizeof less than 200 micrometers. The blend was loosely charged into asuitable microwave transparent oxide ceramic crucible 201, atop a bed ofgranular char to allow for pooling of liquid metal products beneath thereactants. A further layer of granular char was laid over thechromite/coal char blend charge to assist with protection fromre-oxidation. As per Examples 1 and 2, the crucible 201 was insulatedwith an aluminosilicate fibre insulation wrap 202 and a lid 202 aconfigured to allow limited observations through the viewing port 220and allow gaseous reduction products to escape.

The insulated crucible 201 was placed into the reaction chamber 203, inthe form of a modified commercial microwave oven on a ceramic brick 204,“charged” with air at atmospheric pressure. No flushing or evacuationwas carried out.

The reaction chamber 203 was then irradiated with 2450 MHz microwaveradiation at full 1300 W oven power via top and side mounted waveguides212. The reactant charge in the crucible 201 heated rapidly due tomicro-arcing between char particles and then between dielectricallydifferent particles in the charge blend, leading to the onset of somechemical reactions of lower activation energy requirements. The releaseof energy from these initial exothermic reactions provided furtherthermal energy to further heat and activate reduction reactions.

The initial micro-arcing, in the applied microwave field, of the char inthe oxygen containing air environment generated CO, according toEquation 3(a) below, providing protection against re-oxidation ofsubsequent reduction reaction product:2C+O₂

2CO

  3(a)

As the reactant charge increased in temperature, with massive deviationsin local temperatures across a random temperature profile, anon-equilibrium nitrogen plasma was generated in the principallynitrogen (air) atmosphere of the reaction chamber 203, with heatingsubsequently becoming more even throughout the reactant charge. With thehighest temperatures being established in and above the reactant chargewithin the crucible, the plasma was concentrated within the upper levelsof the reactant charge (through plasma penetration of the staticcharge), and directly above the reactant charge within the crucible, 201below the insulating lid 202 a. The radiation, and the plasma, penetratedeeper into the static in-crucible charge with increased permeability ofthe charge.

The nitrogen plasma stimulated initiation of the solid state reductionof the chromite utilising the charcoal as initial direct reductant,producing carbon monoxide (CO) as a by-product of this solid statereduction and which ionises, contributing to the plasma chemistry. Thereactions produce chromium metallic product, leaving a wustite (FeO)rich phase to be reduced in a second stage. This result may be explainedby the greater microwave susceptibility of Cr₂O₃ than FeO (to heat in amicrowave field). The initial solid state reduction of the chromite canbe represented by Equation 3(b) below:FeO.Cr₂O₃

3C

2Cr+FeO+3CO

  3(b)

The CO produced from Equations 3(a) and 3(b) is not thermodynamicallystable below approximately 950° C. when in an environment containingoxygen, such as that of the present example, and, on cooling, tends tooxidise with the oxygen present in the air atmosphere to carbon dioxide(CO₂), according to Equation 3(c) below:2CO+O₂

2CO₂

  3(c)With increasing temperature, however, at the “temperatures” experiencedin the plasma, the inverse is generally true, with free oxygen andcarbon dioxide gas molecules existing in the atmosphere of the reactionchamber 203 directly above the reactant charge being thermochemicallypredisposed towards conversion (with char) to carbon monoxide, accordingto Equations 3(d) and 3(e) below:O₂+2C

2CO

  3(d)CO₂+C

2CO

  3(e)

The CO produced by these reactions itself ionises in the microwavestimulated environment to augment the predominantly nitrogen plasma withhighly energetic, reducing CO⁺ ions. In the reaction chamberenvironment, the plasma enveloping the reactant charge at the higherreducing “temperatures” is accordingly composed primarily of N₂ and COspecies. The plasma protects the charge from possible oxidationreactions to the plasma extremities, maintaining a blanket of highenergy reducing ions over the charge providing a highly reductiveenvironment. Such protection is provided for by the nature of plasmas,and particularly non-equilibrium plasmas, the “chemical emphasis” ofwhich are to break bonds in a manner analogous to chemical reductionreactions (that is, opposite to chemical oxidation reactions where bondsare completed).

The shift in plasma chemistry with the generation of CO⁺ ions could bevisibly observed as a shift in the characteristic emission colour of theplasma and audibly detected by an associated shift in power drawn at themagnetrons where there is plasma initiation or step-augmentation.

The CO available in the ground, excited, ionised and recombined statesbecomes an effective reductant, transferring carbon to the reactioninterface with the chromite particles, initiating a gaseous phasereduction of the chromite. At this stage the reduction rates increase toa maximum. The ionisation of the CO and the ionised gaseous phasereduction of the chromite can be represented by equations 3(f) and 3(g)below:CO

CO⁺+e⁻  3(f)FeO.Cr₂O₃+3CO⁺+3e⁻

2Cr+FeO+3CO₂

  3(g)As discussed above, the CO₂ produced will tend to CO (according toEquation 3(e)) at the high plasma “temperatures” experienced at thisstage. The subsequent solid and gaseous state reduction of the wustite(FeO) product of Equations 3(b) and 3(g) to metallic iron can berepresented by Equations 3(h) and 3(i) below:FeO+C

Fe+CO

  3(h)FeO+CO⁺+e⁻

Fe+CO₂

  3(i)

Where desired, other initial reductant gas (typically CO) can beintroduced preemptively to the reaction chamber 203 via the primary gasinlet 211 to assist the various reduction processes. This will provide areductive gas environment in the reaction chamber from the onset ratherthan relying on conversion of the O₂ within the air environment to CO asdiscussed above.

Hot gases, including reaction by-products CO and CO₂, plus minor andtrace gases, dust and fume were extracted during the process via a fumehood 216 communicating with an exhaust gas-handling system.

During the reduction process, metallic reduction product reported in theliquid state as liquid metal beads, and as the individual beads grew insize and surface tension was overcome, the liquid metallic phase flowedto the base of the crucible 201 forming a pool 230 beneath a slag phase231 of gangue products, which itself formed below the still reacting bedof reactant charge 232 until the depleting solids of the reactant chargebed 232 melted into the liquid slag phase 231. At this point, themicrowave irradiation was ceased and the process terminated.

The reaction chamber 203 was then allowed to cool with the metalreaction product and slag phases solidifying enabling mechanicalrecovery. Passive, slow cooling beneath glowing char (to conferprotection from oxidation) produced a “grey” alloy iron, whilst thealternative cooling process of cooling in water produced a “white” ironalloy.

To avoid oxidisation of the metallic product during cooling as thetemperature within the crucible drops below the CO stability point ofapproximately 950° C. at which the CO would oxidise to the oxidising gasCO₂, a non-oxidising or inert gas can be introduced to the reactionchamber through the primary gas inlet 209 during cooling. Spectroscopicanalyses and metallographic examination of the metallic reaction productidentified a chromium iron alloy with a typical composition ofapproximately 65 at % Cr (and up to 76 at % Cr in minor iron beads),less than 4 at % C, and the balance principally Fe. The C intakeincreases with extended time at elevated temperature. All mineral matterwas converted either to metal or slag, with only remnant char remainingabove the slag phase.

EXAMPLE 4

This example details a process to reduce cassiterite (SnO₂) concentrateto extract metallic tin as product. Rather than being carried out in afixed crucible within a static vessel or a modified microwave oven asper Examples 1 to 3, in this example the reduction process was carriedout in a fluidised bed reactor, utilising a carbon monoxide/nitrogenplasma. The fluidised bed reactor configuration is depicted in FIGS. 5(a) to 5(d). The plasma reduction process can also be carried out inother established and hybrid reaction chamber configurations, includingrotary kiln, cyclone, conveyor strand, screw and launder configurations,using the same basic process chemistry.

The process was conducted at blast-ambit “atmospheric” pressure high inthe bed to higher pressures at the fluidising plate (between 200 kPa and300 kPa) where initial reduction processing may be conducted via appliedmicrowave energy supplied through the reactor base waveguide. Pressuredrop through the fluidised bed is dictated by fluidisation dynamics ofthe reaction chamber and various parameters of the bed being fluidiseditself, including the bed height, particle density, shape and sizerange. A non-equilibrium plasma was sustained along the full height ofthe reaction chamber column with the assistance of supplementarywaveguides along the length of the reaction chamber, the application ofwhich will be dependent upon mineral density, charge susceptibility tomicrowave radiation and applied power. With the pressure drop throughthe fluidised bed, the non-equilibrium plasma was more stable towardsthe top of the reaction chamber.

The current example carried out the reduction processing of a “lowgrade” cassiterite concentrate, containing approximately 60% Sn. Using,traditional reduction techniques for Sn, using reverberatory furnacesmelters, grades below 65% Sn are undesired as it is not economicallyfeasible to process tin product, with the ratio of “hardhead” (iron/tinphase) to recovered tin being too high. When iron is readily reducedwith the tin producing the iron/tin hardhead phase, or the ratio of ironin the initial concentrate is high, the traditional reduction techniquesare typically commercially untenable due to the excessive cost ofextracting tin from the iron/tin hardhead phase. The ease of reductionof cassiterite concentrates increases with increasing tin content fromlow grade to high grade concentrates, with higher grade concentrateshaving been found to be more susceptible to microwave radiation thanlower grade concentrates.

Fluidised bed reactors are commonly configured to carry out continuousprocessing operations, however the present process was carried out in afluidised bed reactor configured for and operated as a batch process toenable tighter control over the cassiterite processing times within thereaction chamber. Such tighter control when processing cassiterite isdesired to avoid over-processing of the cassiterite charge which wouldbe detrimental to post processing operations. The continuous fluidisedbed process is preferred, however, when less control is required on thereduction exposure to the applied electromagnetic radiation.

When the plasma process of the present example is utilised with eitherbatch or continuous fluidised bed systems, the simultaneous reduction ofgangue materials within the ore, particularly ferruginous minerals, isavoided. This consequently avoids the formation of contaminant phases(particularly hardhead, FeSn₂) within the reduction product and theassociated restrictive cost penalties of re-processing such by-productsin subsequent operations.

The fluidised bed reactor 300 of FIGS. 5( a) to 5(d) utilised in thepresent process comprises an elongated reaction chamber 301 formed ofhigh temperature strength, corrosion resistant alloy steel. The reactionchamber 301 is insulated with a suitable refractory insulation wrap 302to maintain heat within the reaction chamber. An air gap may be formedbetween the outer wall of the reaction chamber 301 and the insulationwrap 302 to isolate vibration and accommodate expansion.

A reactant charge inlet 303 is provided at the top of the reactionchamber 301 for charging the reaction chamber 301 with the particulatereaction charge. Referring to FIG. 5( b), a retractable, self sealingcharging bell 304 is disposed within the reactant charge inlet 303. Thecharging bell 304 is rotatable and is provided with spiralling vanes 305to assist in charge distribution within the reaction chamber 301. Otherforms of device for charging the reaction chamber, such as a rotatingchute, may alternatively be employed.

A perforated fluidising plate 306 is located at the base of the reactionchamber 301. A fluidising wind box 307 is located below, and opens onto,the fluidising plate 306. The fluidising wind box 307 communicates witha gas regeneration system (described below and depicted in FIG. 5( c))upstream supplying gas to the wind box 307. The composition and pressureof blast gas supplied to the wind box 307 is controlled by a monitoringsystem 308.

An exhaust outlet 309 is located adjacent the charge inlet 303 at thetop of the reaction chamber 301. The exhaust outlet 309 feeds afume/solids product extraction system (not depicted) to separate fumeand solids fines product from exhausted gases. This system then recyclescooled de-fumed gases into the chamber 310 of the gas regenerationsystem (see FIG. 5( c) via a recycle outlet 311. The chamber 310 isfurther provided with a fresh gas inlet 312 for introduction of gasesfrom outside of the regeneration system, and a pressure relief valve andoutlet 313 for the escape of gases under excess pressure.

A discharge chute 314 communicates with the reaction chamber 301directly above the fluidising plate 306 for the discharging of batchprocess charges upon completion of processing of each batch. The chute314 is closed during processing and communicates with a quenchingchamber 315 for cooling/quenching of discharged material.

Top and bottom waveguides 316, 318 are located at the top of thereaction chamber 301 adjacent the reactant charge inlet 303 and at thebottom of the reaction chamber adjacent the fluidising plate 306respectively. The top waveguide 316 is positioned to irradiate the topregion of the reaction chamber, where off-take gases produced byreactions in the reaction chamber will be present. The bottom waveguide318 is positioned to irradiate the bottom region of the reaction chamberabove the fluidising plate 306. It is at this region that the reactionchamber pressure will be greatest. Further supplementary waveguides 317were located around the periphery of the reaction chamber 301, spacedbetween the top and bottom, and about the circumference thereof (seeFIG. 5( d)). The number, radiating frequency and arrangement ofwaveguides is dependent on the specific application, and in particularwill depend on the reaction chamber configuration and thecharacteristics of the charge being processed. Microwave irradiation at2450 MHz, total power variable up to 100 W per port, was utilised in thepresent example. The general location and orientation of thesupplementary waveguides 317 depicted in FIG. 5( d) is preferred formultiple waveguides positioned along the reaction chamber 301. Ratherthan delivering the radiation utilising waveguides, coaxial cables orany other suitable delivery means could be employed.

In the process of the present example, the cassiterite concentrate wasfirst prepared in particulate form with a grain size of less than 500micrometers and in batches of close size ranges (+/−5% in graindiameter), which are preheated to 200° C. in preparation for charginginto the reaction chamber 301.

Preheated air was then passed, via the wind box 307, through thefluidising plate 306 into the closed reaction chamber 301 and outthrough the exhaust gas outlet 309. Once the reaction chamber proper hadreached approximately 250° C., the preheated air blast was replaced byan N₂/CO mixture (at an N₂:CO ratio of approximately 4:1) preheated toapproximately 300° C. charged into the system from the fresh gas inlet312 of the gas regeneration system. The CO gas was added to form theionising reductant for the reduction of the cassiterite.

Whilst flushing the reaction chamber 301 with the N₂/CO mixture, thepre-heated cassiterite concentrate was charged into the reaction chamber301 through the reactant charge inlet 303 until a full charge wasachieved, and taking care during charging to adjust blast pressure ofthe N₂/CO mixture such that the incoming charge material did not sievethrough the fluidising plate and that the fine charge material was notblasted out of the chamber with the exhaust gases. During charging, thecharging bell 304 was manipulated to regulate and distribute thereactant charge, and in combination with regulation of the N₂/CO blastpressure passing upwards into the reaction chamber 301 through thefluidisation plate, fluidisation of the charge was established andmaintained, whereby the fine charge was maintained in a turbulentsuspension or “fluidised bed” 320. This fluidised bed of reactantparticles maximises the reaction interface between the cassiteritecharge and the gaseous CO reductant.

The fluidisation regime was established such that the fluid bed wassufficiently stable and dielectrically incoherent to allow penetrationof microwave irradiation from the various waveguides 316, 317, 318.

To establish and maintain such a fluidisation regime which is stable andallows penetration of the radiation, the fluidising gas stream throughthe fluidising plate 306 should be incident at the bed base at such apressure as to force the gas, lifting and fluidising the reactant chargebed, towards the zone of lower pressure at the bed stockline (topsurface of the bed toward the top of the reaction chamber), whereoperating pressure should be as low as or close to atmospheric pressureas possible (in the absence of a vacuum evacuation system) so as tominimise the required fluidising pressure. The pressure drop between thefluidising plate 306 and the stockline (which should be minimised) isdetermined by the fluidisation dynamics of the bed in particular theparticle size, density, size range, bed permeability and viscosities,and by the height of the bed. The fluidising pressure at the fluidisingplate 306 is determined by providing the desired fluidising regimewhilst minimising the top pressure at and above the bed stockline. Thispressure at the fluidising plate 306 is controlled by the pressuresustained in the wind box 307. The wind box pressure must equate to thefluidising pressure plus the pressure drop across the fluidising plate306, and was monitored and controlled by the gas re-generation system310 and the feedback control system 308.

The process pressure range, of which the fluidisation pressure at thefluidisation plate will be the maximum, should be kept below 300 kPa tomaintain the plasma within the non-equilibrium thermodynamic regime tomaximise the processing benefits of highly energetic reactive particles,particularly those particles taking a direct role in the plasmareduction chemistry.

The reaction chamber was then irradiated with microwave radiation at2450 MHz frequency via the waveguides, and the power adjusted until astable nitrogen/carbon monoxide non-equilibrium plasma was formed tofull charge height with predominant CO⁺ reactive ions. The common 2450MHz microwave frequency used for this and other examples described wasfound to be highly suited to the applications, and was used primarilyout of convenience. Other frequencies within the range of 30 MHz to 300GHz, radio frequency though microwave and into the “millimetrewavelength” frequencies have, however, been found to be variously wellsuited to the generation of non-equilibrium plasmas preferring anexploitable range of target mineral susceptibilities.

The fluidised cassiterite was reduced by the gaseous CO in the variousstates (ground, excited and ionised), to form Sn and CO₂. The plasmaphase reduction can be represented by equation 4(a) below (where *represents a non-ground state re-combined particle or species):SnO₂+2CO⁺+2e

Sn+2CO₂*   4(a)

Once the plasma chemistry was stabilised, fine carbon was injectedthrough the fresh gas inlet 312 to mix with the fluidising blast gas(N₂/CO), such that CO was regenerated from the CO₂ generated during thegaseous phase reduction of cassiterite, thereby ensuring a continuingsupply of CO⁺ ions for the ongoing reduction of the cassiterite. Thisreaction, known as the Boudouard reaction, is represented by equation4(b) below:C+CO₂*

2CO  4(b)

The Boudouard reaction is endothermic, and hence should only be employedwhen “temperature” moderation is appropriate. This reaction will alsoonly proceed effectively at temperatures above about 940° C. Where, as aresult of the above, the Boudouard reaction is not tenable, CH₄ can beutilised both as a reductant (both directly as methane or indirectly attemperatures above that at which methane decomposes to carbon plushydrogen) to reduce the cassiterite ore and to regenerate CO for furtherreduction. Alternatively, CH₄ can be utilised both for regeneration ofCO by reduction of CO₂ during the process and as a partial or totalreplacement for CO in the initial input gas mixture as initial reductantfor the reduction of the cassiterite. Whilst the CH₄ itself does notionise, dissociating (decomposing) at temperatures below 500° C. in themicrowave field before it reaches its ionisation energy, the resultanthydrogen gas does form a plasma, as will the initial reduction productCO as soon as it is produced, acting as a reductant for subsequentplasma phase reduction. Further, fine carbon soot is produced as aby-product of the methane decomposition, which is ideal forre-generation of CO. Accordingly, even when CO is not used as an initialinput gas, it is soon formed as a reduction by-product and/or throughthe Boudouard reaction breaking down CO₂ at high temperatures. Theaddition of methane to the fluidising gas can also be used to replacethe addition of solid carbon fines to the fluidising gas discussed aboveto enable the CO-regenerating Boudouard reaction 4(b).

At the lower energy or “temperature” ranges of non-equilibrium plasmas,hydrogen is a less efficient reductant than carbon or carbon monoxide.Further, the Boudouard reaction, being endothermic, takes energy fromthe system in supplying CO. Accordingly, selecting CH₄ as initial gasinput is a lower thermodynamic energy option in whose lower energyconditions “tramp elements”, such as Fe, Mn, W and Si which may becontained in the ore reactant charge, will have lower probability ofbeing reduced with the easier to reduce cassiterite, providing a morepure reduction product.

Furthermore utilising CH₄ introduces another gas to the system whichresults in a more complex off-take gas mixture requiring treatment andseparation. The chemical reactions associated with the introduction ofCH₄ can be represented by equations 4(c) to 4(g) below, plus theBoudouard equation 4(b).CH₄

C+2H₂  4(c)SnO₂+2C

Sn+2CO  4(d)SnO₂+2CO

Sn+2CO₂  4(e)2H₂

4H⁺+4e⁻  4(f)SnO₂+4H⁺+4e⁻

Sn+2H₂O  4(g)

Additional chemical reactions of significance with respect to the gasmixture components in the reaction chamber atmosphere (with the additionof methane) can be represented by Equations 4(h) and 4(i) below:2CO+CH₄

2C+2H₂O  4(h)N₂+CO₂+2CH₄

2C+2NH₃+CO+H₂O  4(i)

These reactions are also of significance to the control of off-takegases, minimising negative environmental impact and the recovery ofprocess by-products as materials of value. These reactions take place inthe reaction chamber and, where desired, may be extended to completionin an augmentative chamber of the exhaust outlet 309. Becausestoichiometry is preserved, Equations 4(h) and 4(i) have beengeneralised here in the ground state form for simplicity. Carbonmonoxide and nitrogen will be ionised, whilst (if not alreadydissociated) methane will dissociate to soot plus hydrogen which willionise. Also, other reactions and outcomes are possible but less stable,and thus unlikely.

During processing, off-take gases were exhausted through the exhaustoutlet 309. The exhaust can then be drawn off to a cyclone to separateand remove entrained solid fumes from hot off-take gases bound forscrubbing or recycling (re-generation).

Reduction of the cassiterite produces tin (as indicated in equation4(a)) in the form of micro liquid beads which form within thecassiterite particles and on the particle surfaces, where they are heldtightly by the inherently high surface tension of liquid tin plus a filmof higher melting point material being re-fused or reduction by-productsof gangue minerals.

At a processing point determined by experience and an analysed mean ofthe accumulated data (charge vs time vs applied energy) with respect todegree of reduction of all prior process batches, microwave irradiationof the reaction chamber 301 was ceased. The particulate matter of thestill fluid bed was then cooled to about 200° C. in a non-oxidisingblast gas (nitrogen was used in this example) to solidify the tin. Thesolid contents of the bed were then discharged from the reaction chamber301 through the discharge chute 314 into the quenching chamber 315 forfurther oxygen-free quenching (where required).

The metallic tin content can then be recovered from the quenched solidscontent by a suitable electrochemical or other recovery process. Afterrecovery of the high purity tin, remaining incompletely reducedcassiterite or other mineral particulate material can be dried andreturned for re-processing through the fluidised bed reactor in asubsequent charge blend. Any other fines remaining from the tin recoveryprocess can be subjected to further extraction processes to extract anyremaining high-value metallic content (or toxic content requiringseparation and disposal) which may include metals such as Au, Ag, Th,RE's, Ta, W or Bi.

EXAMPLE 5

In the extractive reduction of comparable metal sulphides of the formMS₂, the “first” sulphur atom can be stripped by reduction with relativeease to yield the matte form, MS. Typically, in a second reductionstage, more intense pyrometallurgical operations are required to removethe remaining sulphur to produce the primary metal.

This example details the reduction of molybdenite (MoS₂) ore concentratein the solid state to yield a crude sponge molybdenum metal—apparently“sintered” by lower melting point phases (gangue and impurity metals).The reduction to metal was achieved in a continuous single stageoperation which utilised the pneumatics of a plug flow fluidised reactorto moderate and balance the applied electromagnetic energy and equablystimulate reactions with an even distribution of energy through thedescending column of charge material. The apparatus utilised to carryout the process is depicted in FIG. 6.

The plug flow fluidised bed reactor 400 utilised was configured forcontinuous processing. In such a reactor, blast gas is directed from ablast box 407 into the reaction chamber 401 though a perforatedfluidising annulus 406 which takes the place of the fluidising plate ofExample 3. In contrast to the batch configured reactor 300 of Example 3,reactant charge material is intermittently or continuously charged viathe charging bell 404 (or a rotating chute equivalent), is fluidised bythe ascending blast gas, the descending fluid bed obeying the mechanicsof plug flow. The plug flow solids are subject to the reactions of theprocess to completeness, before being passed out of the reaction chamber401 through the open discharge funnel 414 in the centre of thefluidising annulus 406 into a quenching chamber 415. Reactant chargematerial is continuously charged into the reaction chamber 401 andreaction product solids continuously discharged without closing down thereactor.

In the present example, the molybdenite ore concentrate was prepared inparticulate form with a grain size of less than 200 micrometers, andblended with a solid reductant in the form of granular charcoal of sizerange 100 to 1200 micrometers in the stoichiometric ratio of 2:1 C:S.Selection of granular charcoal in this larger size range was imposed toprovide better bed permeability given the plate-like morphology ofmolybdenite. The molybdenite/charcoal blend charge was then preheated toapproximately 300° C. in preparation for charging into the reactionchamber 401.

At the beginning of the continuous process, preheated air was flushedthrough the reaction chamber 401 via the wind box 407 and fluidisingannulus 406 into the reaction chamber 401 proper and out through theexhaust gas outlet 409. Once the reaction chamber temperature hadreached approximately 300° C., the preheated blast air was replaced by amixture of 10% CO in air which had been preheated to approximately 300°C., and, upon process start, increased to 600° C. at a rate of 10° C.per minute.

Whilst flushing the reaction chamber with the CO/air mixture at lowblast pressure such that solid fines were not entrained and exhaustedwith the off-take gases, the preheated molybdenite/carbon charge blendwas charged into the reaction chamber 401 (at the calculated rate ofcharging for the continuous operation) via the reactant charge inlet 403and charging bell 404. An initial charge of the charge blend formed atemporary discharge funnel plug in the discharge funnel 414 between theclosed discharge control valve 419 and the first charge of materialabove the fluidising annulus 406 subjected to full processing once acontinuous plug flow had been initiated. Once continuous plug flow hadbeen initiated, the discharge control valve 419 was opened. The firstexiting unprocessed and under-processed material was removed from thequench chamber system and returned for blending with fresh blendmaterial. Whilst the reaction chamber column was reaching full charge,and during the start-up stage, the newly fluidised bed was monitored soas to establish and preserve the plug flow regime which optimises fullmetallurgical conversion (by analysis) versus mean residence time (inthe reaction chamber). The fluidised bed regime was established withsufficient fluid bed stability and dielectric incoherence to allowpenetration of microwave radiation from the various waveguides 416, 417,418 placed at the top, sides and base of the reaction chamber 401 in asimilar manner to Example 3.

Reaction chamber top pressure (pressure above the bed stockline) shouldbe as close to one atmosphere as is possible (given the pressure dropthrough the bed from the pressure at the base required to maintain thefluidisation regime), enabling the fluidisation pressure at the base ofthe reaction chamber to be maintained below the preferred 300 kPa islimit.

Although short wavelength electromagnetic frequencies in ranges above 12GHz would have been preferred, as a result of the enhancedsusceptibility of molybdenite at these frequencies indicated by resultsof mineral susceptibility vs irradiating frequency analyses, radiationat the common frequency of 2450 MHz was used out of availability andconvenience (and found to be adequate for the purpose). Once the chargein the reaction chamber 401 had stabilised near the blast temperature ofapproximately 600°, the reaction chamber was irradiated with microwaveradiation via the various waveguides 416, 417, 418. The power appliedwas adjusted until a stable plasma was formed with predominant CO⁺reactive ions in the nitrogen plasma. The CO⁺ ions were formed followingconversion of O₂ and CO₂ in the presence of charcoal, as the reactionchamber temperature increased beyond 950° C. as discussed in earlierExamples, with the reaction chamber “temperature” increasing to theideal plasma reduction “temperature” of 1050° C. to 1100° C. (asmeasured by shielded thermocouple).

Whilst the exact reduction route achieved is not simple, the addition ofsmall quantities of lime (CaO) to the reactant charge blend, orpelletised molybdenite/lime/brown coal paste dried pellets of close sizerange, here 1.5 mm±0.1 mm to 3.0 mm±0.2 mm, which were ideal for bedpermeability and reduction chemistry with increased kinetics, had theeffect of assisting reduction kinetics and reaction completeness.

Prominent reduction reactions which are understood to have occurredduring processing are represented (without ionisation equivalents) inEquations 5(a) to 5(d). Equations 5(a) represents the initial strippingof the first sulphide atom from MoS₂ and the subsequent reduction toelemental molybdenum being represented by Equation 5(b):MoS₂+O₂

MoS+SO₂  5(a)2MoS+C

Mo+CS₂  5(b)CS₂+2O₂

C+2SO₂  5(c)

Equations 5(d) and 5(e) represent the alternative route when lime isadded:MoS₂+2CaO

MoO₂+2CaS  5(d)MoO₂+2C

Mo+2CO  5(e)

Once plasma chemistry has stabilised, fine carbon may be injected withthe fluidising gas mix such that, where required, CO is regenerated fromO₂ and CO₂ (generated during reduction) with the carbon high in thereaction chamber to confer the protection of a reducing atmosphere in asimilar manner to Example 4. As discussed in relation to Example 4, TheCO₂ reduction, the Boudouard reaction, is endothermic and the balance ofreactions in the reactor can be manipulated such that reactor“temperature” profiles can be maintained as was the case for the earlierexample.

Again in a similar manner to Example 4, CH₄ can be introduced both toregenerate CO and to provide an extra control mechanism (in addition tocontrol of the applied electromagnetic radiation) by balancing thechemical energy released by exothermic reactions and that absorbed byendothermic reactions within the reaction chamber. The various reactionsresulting from the addition of CH₄ are as per those of Equations 4(c),4(b), 4(f), 4(h) and 4(i) discussed in relation to Example 4. It shouldbe noted that whilst hydrogen increases its efficiency as a reductant inthe higher operating temperatures of the present process, it does noteclipse carbon monoxide in reduction efficiency until much highertemperatures (above 2000° C.). Furthermore, the dissociation of methaneto provide active hydrogen will result in the production of hydrogensulphide (H₂S) gas which is normally a less desirable offtake gasoption. The additional reduction reactions resulting from the generationof hydrogen ions through the addition of methane can be represented byEquations 5(f) and 5(g):MoS₂+2H⁺

MoS+H₂S  5(f)MoS+2H⁺

Mo+H₂S  5(g)

At the base of the reaction chamber 401, the loose, fluid productparticulate solid of the descending bed was discharged through thedischarge funnel 414. The rate of descent in the bed was controlled bythe rate of discharge of processed solid material through the dischargefunnel 414 which is governed by the setting of the discharge controlvalve 419. Reaction chamber residence time is dictated by rate of plugflow descent (of charge elements), which (given unhindered particulatefluidity) is controlled by the discharge rate, which is regulated by thedischarge control valve setting. Residence time is determined by thethermochemical processing parameters (such as chemical availability,contact and reaction interface diffusion, chemical species, availableenergy and energy required) and the physical and chemical kinetics whichdetermine the overall rate of chemical conversion, thence the timerequired for chemical conversion. The time required for chemicalconversion should ideally be slightly less or equal to the designatedresidence time of reactant material in the reaction chamber.

The discharged material entered the quenching chamber 415 where it waskept mobile during cooling to minimise agglomeration of particles and toprevent bulk “sintering”. The particulate solid product is in a formwhich can be easily managed and handled, and may be sent for furtherrefining stage processing such as an arc or ion beam melt, zone refiningoperation.

As product was continuously discharged from the fluidised bed reactionchamber (column) 401, fresh charge blend material was continuallycharged onto the stockline of the fluidised bed in an even manner suchthat the stockline level remained constant.

EXAMPLE 6

This example details a process to reduce haematite (Fe₂O₃) using aconveyor to pass reactant charge material through an atmosphericpressure reaction chamber in a continuous process. The apparatus, termeda continuous conveyor fed reactor, utilised to carry out the process isdepicted in FIG. 7.

Fine haematite was blended with a reductant in the form of fine browncoal char, the mixture was bound into a paste using brown coal slurry toresult in an Fe:C ratio of approximately 2:3. The paste was agglomeratedinto pellets of approximately 3 millimetre diameter and dried untilhard.

The dried pellets 501 were then evenly distributed across a sinterstrand type conveyor 502 configured to allow blast gases to passtherethrough.

The pellets 501 on the conveyor 502 were then passed through thereaction chamber 503 of the continuous conveyor fed reactor. Thereaction chamber 503 is configured with an inlet choke region 503 a ofrestricted cross section, an open main chamber region 503 b and anoutlet choke region 503 c of restricted cross-section. The dried pellets501 were first irradiated with microwave radiation via a preliminarywaveguide 504 in the inlet choke region 503 a at a frequency ofapproximately 915 MHz, providing preliminary heating of the pellets.This preliminary heating may bring the haematite/char reactants to above500° C., close to a temperature capable of initiating initial reductionreactions. As the heated pellets pass from the inlet choke region 503 atowards the main open region 503 b of the reaction chamber 503, they areirradiated with microwave radiation from central wave guides 505 at afrequency of 2450 MHz. In this central region, a hot air blast (atapproximately 800° C.) is imparted on the pelletised reactant chargefrom a blast inlet 506 positioned directly beneath the conveyor 502 inthe centre of the main chamber region 503 b. The hot oxygen of the airpassing through the carbon of the (now glowing red) hot reactants of theconveyor charge (at temperatures up to 1000° C.) rapidly converts tocarbon monoxide. An N₂/CO plasma is sustained immediately above thecharge in the region of the blast inlet 506, providing highly energeticreactive species in the primary reduction zone.

The reduction of haematite to elemental iron takes place through aseries of reduction reactions which can be represented by Equations 6(a)to 6(g) below (without ionisation equivalents), general system pressureand temperature, and with respect to the local ionisation environment ofthe plasma zone, the reaction path depending upon the available energyof activation and the reaction mechanism, whether a solid state or gasphase reaction.1.5Fe₂O₃+0.5C

Fe₃O₄+0.5CO  6(a)Fe₃O₄+C

3FeO+CO  6(b)FeO+C

Fe+CO  6(c)1.5Fe₂O₃+0.5CO

Fe₃O₄+0.5CO₂  6(d)Fe₃O₄+CO

3FeO+CO₂  6(e)FeO+CO

Fe+CO₂  6(f)CO₂+C

2CO  6(g)

Offtake gases produced from the reactions are exhausted through theexhaust outlet 507 and treated for heat recovery or blast regeneration.

The CO plasma is positionally maintained in the centre of the reactionchamber as a result of the location of the hot air blast inlet 506 andthe positioning of the main microwave radiation wave guides 505. Thespongy solid reduced iron is subject to cooling as it travels from theplasma zone towards the outlet choke region 503 c. As the iron coolsbelow 950° C., nitrogen may be introduced to the atmosphere of theoutlet choke region 503 c to blanket the conveyor 502 and protect thereduced iron from re-oxidation which may result from free O₂ or from COwithin the cooling chamber environment being converted to CO₂ at thislower temperature regime at which CO₂ exhibits stability.

EXAMPLE 7

This example details another process to reduce haematite to ironutilising a rotary kiln device using the same basic preparation andchemistry as Example 6. The apparatus utilised to carry out the processof this example is depicted in FIG. 8.

Haematite/carbon pellets 601 prepared in accordance with Example 6 werefed into the rotary kiln reaction chamber 603 via a reactant chargeinlet 602. The reaction chamber 603 was charged via gas inlet 606 with agas mixture of 10% CO in N₂ at low velocity, maintaining the pressurewithin the reaction chamber at approximately 1 atmosphere.

The 28 liter reaction chamber 603 was irradiated with microwaveradiation at a power of approximately 1000 watt and frequency of 2450MHz via a wave guide 605 positioned in a stationary end of the kiln,generating a CO/N₂ plasma throughout the central (16 to 20 liter) corevolume (not occupied by the revolving charge nor spiral ribs 609) of therotating reaction chamber 603.

The microwave radiation was moderated to prevent melting of the reactantcharge, as detected and monitored by inspection of reduction productdischarge which was drawn from the reaction chamber 603 via thedischarge outlet 608. Both solid and gaseous phase products of thereaction product were drawn from the discharge outlet 608.

EXAMPLE 8

This example details a process to reduce haematite (Fe₂O₃) to low carboniron (Fe) product, using a principally solid state reduction techniqueof in-flight entrainment of fine particulate charge material in acyclone reactor. The apparatus utilised to carry out the process of thisexample is depicted in FIGS. 9( a) to 9(c).

Firstly haematite was prepared in a particulate form by milling to agrain size of less than 20 micrometers, and intimately blended with finebrown coal char milled to a grain size of less than 100 micrometers.

The reactant blend of haematite/char was then entrained with a blaststream of air preheated to in excess of 400° C. through a cyclone inlet701 located at the top of the cyclone reaction chamber 702. The inlet701 is arranged tangential to the cylindrical upper wall portion of thereaction chamber 702 such that the inlet blast air and entrained chargedmaterial follows a spiral path down through the reaction chamber 702.

The blast air and entrained reactant charge were irradiated withmicrowave radiation at a frequency of 2450 MHz via a primary waveguide703 arranged to irradiate the air and reactant charge as it passed alongthe inlet 701 prior to entry into the reaction chamber 702 proper. Thepower of the microwave radiation applied was controlled to raise theblast air temperature to 1000° C. to ensure that free oxygen in theblast gases reacted with the fine char of the entrained charge blendconverting O₂ through carbon dioxide (CO₂) to carbon monoxide (CO). Theoperating pressure range was kept below the preferred pressure maximumof 300 kPa, such that the N₂/CO plasma formed beyond the port and in thecyclone reaction chamber was in the upper range of non-equilibriumconditions, raising the air to an appropriate temperature to ensure thatremaining free oxygen (O₂) and carbon dioxide (CO₂) in the blast airreacted with the fine char of the reactant charge blend to convert themto carbon monoxide (CO).

The reactant charge and blast air were further irradiated by furtherprimary microwave waveguides 704 at the top of the reaction chamber 702,positioned around the circumference of the reaction chamber 702 asindicated in FIG. 9( b).

Whilst the 2450 MHz frequency utilised in the present example was foundto be effective and efficient in the present example, frequencies inexcess of 12 GHz were found to be preferable for this specificapplication.

In the non-equilibrium N₂/CO plasma environment toward the top of thereaction chamber, the haematite was reduced to magnetite, Fe3O4,utilising the highly energetic, reactive CO⁺ ions for primary reductionstimulation. The magnetite was subsequently reduced to the mostthermodynamically stable of the iron oxide phases, wustite, FeO, whichwas subsequently reduced to metallic iron. Any liquid phase formationfrom exothermic reactions was avoided by the in-flight cooling dynamicswithin the reaction chamber. The reduced product had the appearance ofsolid state reduction rather than re-solidification of a reduced liquidphase. Any tendency to CO₂ generation during the sequence of reductionwas countered by Boudouard gasification between remaining entrained charand CO₂ to re-generate CO (as per Equation 6(g)) in reaction chamber702.

This CO regeneration process was assisted through the addition offurther fine free carbon via the cyclone inlet 701. The CO produced fromthe CO₂ can be utilised for further reduction in the various stages ofthe haematite reduction to iron, and also provides protection againstoxidation of the elemental iron product.

The reactions of generating and re-generating CO are endothermic, andaccordingly further external energy was required to maintain thetemperature within the reaction chamber. This energy was applied throughsupplementary microwave waveguides 705 positioned around thecircumference of the reaction chamber 702 and spaced therealong.

A typical circumferentially spaced pattern of supplementary waveguides705 is depicted in FIG. 9( c). The supplementary waveguides 705 alsoprovided the energy required for the final reduction stage of the FeO tometallic iron, which required considerably more is energy than theinitial reduction stages.

The reduced iron particles cooled as the spiral path passed into thelower tapered portion of the reaction chamber 702 below the effect ofthe various waveguides, and as a result of the mass of the ironparticles disengaged from the entrained flow and dropped into acollection chamber 706 at the base of the reaction chamber 702. Thecollection chamber was blanketed with CO (or alternately inert gas) toprotect the reduced iron from oxidation. The solid iron productretrieved from the collection chamber 706 was in the form of high carbonsteel powder, which was relatively low in carbon (0.3 wt % to 1 wt %compared to iron).

Off-take gases from the various reactions were extracted from theexhaust gas outlet 707 located in the centre of the top of the reactionchamber 702, and diverted to appropriate gas handling systems (baghouse,scrubbing, gas regeneration as appropriate).

The off-take gases primarily consisted of N₂, CO and CO₂.

The person skilled in the art will appreciate the manner in which thevarious forms of reaction can be applied utilising the radiationstimulated plasma of the present invention.

1. A process for the reduction of a metalliferous ore or concentratecomprising the steps of: preparing said ore or concentrate into aparticulate form; charging a reaction chamber with said ore orconcentrate, a reductant and an input gas; irradiating said reactionchamber with electromagnetic radiation within a frequency range of 30MHz to 300 GHz until a non-equilibrium plasma is initiated, andsustaining and controlling said non-equilibrium plasma with saidradiation until said ore or concentrate is reduced to form reductionproduct; wherein pressure within said reaction chamber is maintainedabove 40 kPa and below 300 kPa during irradiation thereof; and furtherwherein said reductant comprises a carbonaceous material.
 2. The processof claim 1 wherein said pressure is maintained at about atmosphericpressure.
 3. The process of claim 1 wherein said plasma is initiated insaid input gas.
 4. The process of claim 1 wherein at least part of saidinput gas is decomposed during said irradiation, said plasma beinginitiated at least in part in the decomposed product of said input gas.5. The process of claim 1 wherein said reductant includes a particulatecarbonaceous material blended with said ore or concentrate.
 6. Theprocess of claim 1 wherein said reductant includes carbon monoxide gas,said input gas including said carbon monoxide gas, said plasma beinginitiated at least in part in said carbon monoxide gas.
 7. The processof claim 1 wherein said reductant comprises carbon monoxide gas and aparticulate carbonaceous material.
 8. The process of claim 1 whereinsaid reductant comprises a reactive metal.
 9. The process of claim 1wherein said input gas includes an inert gas.
 10. The process of claim 9wherein said inert gas comprises argon or nitrogen.
 11. The process ofclaim 1 wherein said input gas comprises air.
 12. The process of claim 1wherein said reductant includes methane.
 13. The process of claim 1wherein said radiation is microwave radiation.
 14. The process of claim1 wherein said ore or concentrate is a concentrate derived directly frommined ore.
 15. The process of claim 1 wherein said ore or concentrate isa non-ore derived concentrate.
 16. The process of claim 1 wherein saidore or concentrate is a concentrate in the form of a residue derivedfrom metallurgical processing operations.
 17. The process of claim 1wherein said reaction chamber is in the form of a fluidised bed reactor.18. The process of claim 1 wherein said reaction chamber is in the formof an oven, said ore or concentrate being charged into a crucible placedwithin said oven.
 19. The process of claim 1 wherein said reactionchamber is in the form of a rotary kiln reactor.
 20. The process ofclaim 1 wherein said reaction chamber is in the form of a cyclonereactor.
 21. The process of claim 1 wherein said reaction chamber is inthe form of a conveyor fed reaction.
 22. The process of claim 21 whereinsaid ore or concentrate is prepared into a pelletised particulate form.23. The process of claim 1 wherein said reduction product is of metallicform.
 24. The process of claim 23 wherein said metallic reductionproduct is in the form of a fume, said fume being extracted from saidreaction chamber and separated from gases produced during saidreduction.
 25. The process of claim 1 wherein said reduction product isa compound of reduced oxidation state.
 26. The process of claim 1wherein said reduction product is formed by reduction of said ore orconcentrate through a series of subsequent reduction reactions.
 27. Theprocess of claim 1 wherein said process includes the step of generatingcarbon monoxide, said plasma being initiated and sustained at least inpart in said carbon monoxide.
 28. The process of claim 27 wherein saidinput gas includes air, said reductant includes particulate carbonaceousmaterial and said carbon monoxide is generated from reaction of oxygenwithin said air with said particulate carbonaceous material.
 29. Theprocess of claim 27 wherein said reductant includes particulatecarbonaceous material and said carbon monoxide is generated fromreaction of carbon dioxide produced during said reduction with saidparticulate carbonaceous material.
 30. The process of claim 27 whereinparticulate carbonaceous material is introduced into said reactionchamber after initiation of said plasma, said carbon monoxide beinggenerated from reaction of carbon dioxide produced during saidreduction, with said introduced particulate carbonaceous material. 31.The process of claim 27 wherein said input gas includes air andparticulate carbonaceous material is introduced into said reactionchamber after initiation of said plasma, said carbon monoxide beinggenerated from reaction of oxygen within said air with said introducedparticulate carbonaceous material.
 32. The process of claim 1 whereinsaid ore or concentrate is enveloped in a non-oxidising or inert gasenvironment during said reduction and during cooling of said reductionproduct following irradiation of said reaction chamber.
 33. The processof claim 32 wherein said non-oxidising or inert gas is introduced tosaid reaction chamber during said cooling.
 34. The process of claim 1wherein said input gas is passed through said ore or concentrate duringsaid irradiating step.
 35. The process of claim 34 wherein said inputgas is blasted upwardly through said ore or concentrate.
 36. The processof claim 1 wherein said input gas is preheated prior to charging intosaid reaction chamber.